An optical device for reading information recorded on an optical disc having a transparent substrate of about of 1.2 mm in thickness, such as a CD, by using a semiconductor laser has been provided. In such an optical pickup device, focus servo and tracking servo control for an objective lens of the pickup device is performed, a pit string on the recording surface is irradiated with laser beam, whereby audio, video and data signals are reproduced.
Referring to FIG. 91, in a recordable compact disc (CD-R) 150 which is one type of CDs, cyanine dye 152 is provided on a recording surface side of transparent substrate 151, and gold 153 is further provided. CD-R has the same track pitch, recording density and pit length as those of a general CD for music, and the CD-R is different from the CD for music only in that it allows one recording. Recording is done by irradiating cyanine dye 152 with laser beam through transparent substrate 151. When cyanine dye 152 is irradiated with laser beam, optical characteristic of the irradiated portion becomes uneven. Accordingly, at a portion not irradiated with laser beam at the time of recording, most laser beam is reflected by gold 153 at the time of reproduction, while at portions irradiated with laser beam at the time of recording, laser beam is hardly reflected at the time of reproduction.
Recently, density has been increased in order to record a motion picture for a long period of time on such an optical disc. For example, a DVD allowing recording of information of about 5 G byte on one side having the same diameter of 12 cm as the CD has been proposed. Thickness of the transparent substrate of the DVD is about 0.6 mm. One DVD formed by adhering two transparent substrates back to back allows recording of information of about 10 G byte.
However, the aforementioned pickup objective lens is designed taking into consideration the thickness of the transparent substrate of the optical disc of interest and wavelength of semiconductor laser used. Therefore, when an optical disc having a thickness different from the design is to be reproduced, laser beam is not focused on the recording surface of the optical disc, and hence reproduction is not possible. For example, an objective lens designed for a CD having transparent substrate of 1.2 mm in thickness is not capable of focusing laser beam on the recording surface of a DVD having a transparent substrate 0.6 mm in thickness.
In CD-R 150 employing cyanine dye 152 mentioned above, maximum reflectance is attained when the laser beam has the wavelength of 800 nm, while reflectance is not higher than 10% when the laser beam has the wavelength of 635 nm, as shown in FIG. 92. Accordingly, reproduction of a CD-R is not possible by the laser beam having the wavelength of 635 nm which is used for reproduction of a DVD. In order to reproduce the CD-R with the laser beam having the wavelength of 635 nm, it may be possible to use dye having higher reflectance to the laser beam having the wavelength of 635 nm in place of cyanine dye 152. However, it is not practical to change CD-R standard to be in line with the DVD.
The present invention was made to solve the above described problem, and its object is to provide an optical pickup device capable of recording and/or reproducing two different types of optical discs having transparent substrate of different thicknesses.